The plugging and unplugging of cards to/from a board raise problems when the cards are not provided with their own power supply but are powered from a common power supply, because a surge current is generated when a card is plugged which disturbs the operation of the circuits located on the already plugged cards.
It is well known to use connectors having different pin lengths to provide for the correct sequencing of the applications of the different voltage levels which are required by the circuits arranged on the cards, when the cards are plugged.
Such arrangements are described in U.S. Pat. No. 3,993,935 and in the article published in the IBM Technical Disclosure Bulletin Volume 31 No. 6- Nov. 6, 1988 page 125.
FIG. 1 shows how the problem of the surge current generated when a card is plugged is solved in a conventional way.
FIG. 1 shows a card 1 called master card because it comprises a power supply device 2 able to provide the supply voltages to cards such as 3 pluggable to card 1 through a connecting arrangement 4 comprising a board and connectors for example. The power supply 2 provides a first voltage at terminal 5 of the connecting arrangement 4 and a second voltage at two terminals 6-1 and 6-2, it will be assumed that the first voltage is the ground voltage and the second voltage is a positive supply voltage +V.
For the sake of simplicity, only one supply voltage +V is shown in FIG. 1, but depending upon the operation of the circuits arranged on the cards several different supply voltages may be required.
The card 3 can be connected to the connecting arrangement 4 by means of male connector 7 which comprises a ground pin 8 and two voltage pins 9-1 and 9-2 which can be plugged to terminals 5, 6-1 and 6-2 respectively. The pin 9-1 is longer than the pin 9-2. When the connector 7 is plugged to connecting arrangement 4, the ground voltage and V voltage are applied for biasing the circuit arranged on card 3 through pin 8 and pin 9-2 and lines 10 and 11. A decoupling capacitor Cd is arranged between the lines 10 and 11 providing the ground and V voltage to the circuit on card 3.
To solve the problem raised by the surge current Is due to the charge of the decoupling capacitor Cd when the card is plugged, a resistor R of resistance r limits the surge current Is to an acceptable value.
The resistance r depends upon the voltage V and upon the value of the surge current which is tolerated. The time constant .tau. of the charge of the capacitor Cd must be low with respect to the time difference between the application of voltage V to long pin 9-1 and the application of voltage V to short pin 9-2, which is a specification of the connector. When the capacitance of capacitor Cd is low, a resistance r can be found which meets the above constraints. But when the capacitance Cd is high the problem is unsolvable.